The present invention relates to a dielectric filter which can obtain preferable spurious suppressing characteristics in a microwave region used in a base station of a portable telephone system and, more particularly, to a dielectric filter obtained by combining dielectric resonators having different frequency characteristics in unnecessary harmonic wave modes to efficiently suppress a spurious generated near a desired pass band.
1. Background Art
In recent years, low-loss filters having high stability are variously used to achieve a reduction in size and an increase in reliability of devices in high-frequency bands. In particular, a dielectric filter using a dielectric resonator is popularly used as a narrow-band and low-loss band-pass filter.
As a dielectric filter of this type using a dielectric resonator, for example, a TE01 mode filter disclosed in IEEE MTT-S INTERNATIONAL MICROWAVE SYMPOSIUM DIGEST WEIF-13 xe2x80x9cHIGH Q TE01 MODE DR CAVITY FILTERS FOR WIRELESS BASE STATIONSxe2x80x9d (issued in 1998) is known. Rough configurations of this filter are shown in FIGS. 13(a) to 13(c).
2. Prior Art
In FIGS. 13(a) to 13(c), separate spaces partitioned at predetermined intervals with electromagnetic coupling windows by partition walls 101A of the same material as that of the case 101 are formed in a shield unit 100 constituted by a cavity metal case 101 for forming a shield housing and a metal lid 102, and a plurality of dielectric resonators 104 to 109 electromagnetically coupled to each other and having the same shapes are formed on support tables 110 in the separate spaces, respectively. Input/output connectors 111 and 112 are attached to one end portion of the shield unit 103, and a probe 113 electromagnetically coupled to the dielectric resonator 104 is arranged on the connector 111, and a probe 114 electromagnetically coupled to the dielectric resonator 109 is arranged on the connector 112. On the lid 102, at the positions of the dielectric resonators 104 to 109, tuning plates 115 to 120 constituted by metal screws and plates are arranged, respectively, and the plate positions are adjusted to thereby adjust resonance frequencies of the respective dielectric resonators. Reference numerals 121 to 125 shown in FIG. 13(a) denote adjusting screws for adjusting electromagnetic couplings, and the adjusting screws adjust electromagnetic couplings between adjacent dielectric resonators.
In the configuration of the conventional dielectric filter described above, by high-order mode resonance, high-level unnecessary resonance (to be referred to as a xe2x80x9cspurious pulsexe2x80x9d hereinafter) is generated at the same frequency outside a passing band to disadvantageously deteriorate filter characteristics. In addition, when a low-pass filter is loaded to suppress the spurious pulse, the low-pass filter requires sharp characteristics, and the number of stages of the filters increases to increase the insertion loss of the low-pass filter. That is, the insertion loss of the entire filter increases. The present invention is to solve the above problem, and has as its object to provide a dielectric filter which can suppress a spurious pulse and can reduce insertion loss.
In order to achieve the object, the present invention provides a dielectric filter in which, as a combination of dielectric resonators having different frequency characteristics in unnecessary harmonic modes, dielectric resonators having at least two types of different shapes or different dielectric constants are arranged in a metal shielding unit, so that an electromagnetic field distribution is changed to efficiently suppress a spurious pulse near a desired passing band.
The first aspect of the present invention is a dielectric filter having a metal case, a lid, and a plurality of dielectric resonators arranged through support tables in spaces partitioned by a metal partition wall inside the metal case and characterized in that the dielectric filter is constituted by a combination of at least two types of dielectric resonators having different frequency characteristics in unnecessary harmonic modes except for a main mode near a passing band of the filter. With the above configuration, a spurious pulse can be extremely effectively suppressed.
The second aspect is characterized in that, in the first aspect, the dielectric resonators having different frequency characteristics in the unnecessary harmonic modes are constituted by a combination of dielectric resonators having at least two types of different shapes.
The third aspect is characterized in that, in the second aspect, the dielectric resonators are dielectric resonators having at least two types of different aspect ratios.
The fourth aspect is characterized in that, in the second aspect, the dielectric resonators are constituted by a dielectric resonator which has an inner hole and a dielectric resonator which has no inner hole.
The fifth aspect is characterized in that, in the second aspect, the dielectric resonators have inner holes having at least two types of different diameters.
The sixth aspect is characterized in that, in the first aspect, the dielectric resonators having the different frequency characteristics in the unnecessary harmonic modes are constituted by a combination of dielectric resonators having at least two types of different dielectric constants.
The seventh aspect of the present invention is characterized in that, in the first aspect, the plurality of dielectric resonators are held by cylindrical support tables having at least two types of different thickness.
The eighth aspect of the present invention is characterized in that, in the first aspect, the plurality of dielectric resonators are constituted by a combination of at least two types of at least two types of different aspect ratios, diameters of inner holes, dielectric constants, and thickness of cylindrical support tables.
The ninth aspect of the present invention is characterized in that, in the first aspect, the metal case forming the dielectric filter and at least one of input/output terminals are connected through a duct integrated with the metal case, and a low-pass filter is formed in the duct. With this configuration, since the dielectric filter and the low-pass filter are connected to each other without a connector, a reduction in insertion loss and a reduction in cost can be extremely effectively achieved.
The tenth aspect of the present invention is characterized in that, in the ninth aspect, the duct has an outer diameter different from the outer diameter of a cable duct used for the input/output terminals.
The eleventh aspect of the present invention is characterized in that, in the first aspect, in the dielectric filter, tuning plates for adjusting the resonance frequencies of the dielectric resonators and a metal rod member are adjustably inserted at remote positions from the dielectric resonators and near the metal case.
The twelfth aspect of the present invention is characterized in that, in the eleventh aspect, the metal rod member is a metal screw which is inserted through a screw hole for connecting the metal case and the lid.
According to the present invention, dielectric resonators having at least two types of different shapes or different dielecrtic constants are arranged in spaces partitioned by a partition wall in a shielding unit constituted by a metal case and a lid, whereby excellent spurious suppressing characteristics can be obtained, and insertion loss can be reduced.